Let us read about reference clocks. Precision of internal clock is classified into so called “Stratum” levels. Accuracy of reference clock is defined as the ratio of bit slip happening (causing a bit error)
Stratum 1 => 1 x 10-11 (synchronization to atomic clock)
Stratum 2 => 1.6 x 10-9
Stratum 3E => 1 x 10-6
Stratum 3 => 4.6 x 10-6
Stratum 4 => 32 x 10-6 (typical for IP routers)
When we are distributing the clock in the network, accuracy level might decrease at each hop in clock distribution. Originally providing Stratum 1 clocks for each network element was far from being economical, even providing this service at multiple locations was too much demanding. So clock distribution methods were developed to minimize the number of high accuracy clocks needed in the network.
Global Positioning System (GPS) includes Stratum 1 atomic clocks on the satellites. Cheap GPS receivers are available in the market and they make it possible to have a Stratum 1 time source at almost any place. This reduces the need for time synchronization network (might even go away in the future…).
Clock Distribution Methods :
Various clock distribution methods are as described below.
When all equipment is at the same location, External clock input might be used. This is usually BITS = Building Integrated Timing Signal. It uses an empty T1 or E1 framing to embed clock signal. Might be provided as a dedicated bus reaching into each rack in a CO environment. BITS should be generated from a Stratum 1 clock. Typically it will be deployed with a hot spare alternative source for fail-over.
Network elements not close to a BITS source should recover clock from the line. While distributing the clock, Clock distribution network should not have loops, so a tree distribution topology should be configured. Usually carrier network element will have Stratum 3 accuracy when running free. By synchronization to the reference clock, this clock is running at the same rate as the reference clock (that is Stratum 1). Minimum requirement for any network element is 20 ppm (that is between Stratum 3 and Stratum 4).
Alternative Clock Sources:
If the trail to the reference clock source is lost, the network element still continues normal operation. However, alarm might be generated. After some time the clock might drift away so much, that bit errors would occur. Some time is left for switching over to an alternative clock source. Then the network element gets into a holdover state. Requirement is to have less than 255 errors in 24 hours.
A hierarchy of potential clock sources should be configured at each network element to achieve a high-availability operation. Typically a maximum 3 alternative time reference sources might be configured. This is meaningful only if there are different paths to the alternative time reference sources. If only one natural path exists to a single time reference source, then the path must be protected by automatic protection switching. This requires some extra signaling to do it properly, called SPS = Synchronization Protection Switching.